1. Field of Invention
The present invention relates to an harmless treatment system for an organic wastewater with a high salinity (wherein an inorganic salt content is 5 wt %-30 wt %), wherein supercritical water is utilized as a reaction medium.
2. Description of Related Arts
The supercritical water is the special-status water whose temperature and pressure are both above critical point of water (wherein T=374.15° C., P=22.12 MPa). The supercritical water has the properties of both liquid and gaseous water, and only a small amount of hydrogen bonds exist under this condition. Therefore, a dielectric constant of the supercritical water is similar to that of the polar organic solvent, and the supercritical water has a high diffusion coefficient and a low viscosity. Organics, oxygen and the supercritical water are soluble in each other, in such a manner that heterogeneous reactions become homogeneous reactions, which greatly reduces the mass transfer resistance. However, inorganic salt has a very low solubility in supercritical water, and is easy to be separated.
Supercritical water oxidation (SCWO) takes advantage of a special property that the supercritical water is a good solvent for both organics and oxidant. With adequate oxidants, organics in an oxygen-rich environment homogeneously react, which rapidly and completely destroys the organics and transforms them into harmless small molecule compounds such as H2O and CO2, and inorganic salts. SCWO is mainly applied in efficiently harmless treatment for organic wastewater with a high toxicity, a high concentration, and a low biochemical degradability, which has no secondary pollution, is able to achieve heat self-sufficiency, costs less in the case of energy recovery and optimization, has economic advantages, and has a promising future in terms of displacing the conventional incineration method. Thus, the development of SCWO attracts the attention of the whole world. “Energy and environment”, as one of the six key areas of the US national technologies, points out that one of the most promising organic waste treatment technology of the 21st century is the supercritical water oxidation technology. Currently, there are a small number of commercial SCWO devices running in some countries, but most are still in experimental stages and only a few pilot scale devices are developed.
Organic wastewater with a high concentration and a low biochemical degradability (such as pesticide wastewater) usually comprises a large amount of inorganic salts up to 5 wt %-30 wt %, wherein some inorganic salts have recycling values. However, the solubility of the inorganic salt is significantly low in the supercritical water, typically less than 100 mg/L. For example, the solubility of Na2SO4, CaCl2, NaCl or KCl in supercritical water at 400° C., 25 MPa is less than 1 g/L. Viscous salts in supercritical water oxidation of organic wastewater will precipitate and deposit on the reactor surface. When the salt deposition is out of control, the reactor will be plugged. Especially, when viscous salts with larger particles precipitate at a low flow rate condition, the reactor is more likely to be plugged. And when this happen, the entire device must be shut down, washed and restarted, which reduces the reliability of the SCWO device and increases its operation cost. In addition, the deposition of the inorganic salts, in particular those containing chlorine ions, will increase the corrosion rates of the reactor, pipes and other parts, and result in heat transfer deterioration of a heat exchanger surface. The environment with high temperature, high pressure and high oxygen content increases the operation cost of the SCWO device. The above problems greatly hinder the application of SCWO.
In view of the complex feedstock characteristics and the harsh reaction conditions during SCWO of the organic wastewater, the conventional desalination methods (such as electrical dialysis, reverse osmosis, ion exchange, electrical adsorption) are difficult to be applied in the SCWO system for an organic wastewater with a high salinity. For the reliable operation of this SCWO system, the simpler, more efficient and more convenient desalination methods and equipments are needed. Therefore, for the development of the SCWO system for the organic wastewater with a high salinity, the reactor plugging caused by salt deposition should be solved, and the operation cost of SCWO should be effectively reduced.